Since Firefox add-ons operate on a live browser DOM, what you’ll actually see
when inspecting the page source is not the original HTML, but a modified one
after applying some browser clean up and executing Javascript code. Firefox,
in particular, is known for adding <tbody> elements to tables. Scrapy, on
the other hand, does not modify the original page HTML, so you won’t be able to
extract any data if you use <tbody> in your XPath expressions.

Therefore, you should keep in mind the following things when working with
Firefox and XPath:

Disable Firefox Javascript while inspecting the DOM looking for XPaths to be
used in Scrapy

Never use full XPath paths, use relative and clever ones based on attributes
(such as id, class, width, etc) or any identifying features like
contains(@href,'image').

Never include <tbody> elements in your XPath expressions unless you
really know what you’re doing

Firebug is a widely known tool among web developers and it’s also very
useful for scraping. In particular, its Inspect Element feature comes very
handy when you need to construct the XPaths for extracting data because it
allows you to view the HTML code of each page element while moving your mouse
over it.

Firecookie makes it easier to view and manage cookies. You can use this
extension to create a new cookie, delete existing cookies, see a list of cookies
for the current site, manage cookies permissions and a lot more.